Method and apparatus for unstructured control flow for SIMD execution engine

ABSTRACT

An apparatus and method for a SIMD unstructured branching. For example, one embodiment of a processor comprises: an execution unit having a plurality of channels to execute instructions; and a branch unit to process unstructured control flow instructions and to maintain a per channel count value for each channel, the branch unit to store instruction pointer tags for the unstructured control flow instructions in a memory and identify the instruction pointer tags using tag addresses, the branch unit to further enable and disable the channels based at least on the per channel count value.

BACKGROUND

Field of the Invention

This invention relates generally to the field of computer processors.More particularly, the invention relates to an apparatus and method forperforming single instruction multiple data (SIMD) unstructuredbranching.

Description of the Related Art

Managing control flow in single instruction multiple data (SIMD)programs is a complex problem. Traditionally, graphics processing units(GPUs) use scalar code and program routines to control instructionpointer (IP) addresses for each SIMD channel. This is inefficient bothin terms of performance and power usage.

Control flow is managed on some architectures by maintaining a unique IPaddress for each channel. For example, when a control flow instructionis encountered, the IP of each channel is updated with a particular IPbased on the predicate mask of the instruction. For each and everyinstruction, the execution IP is compared to the channel's IP todetermine if that channel is enabled for a particular instruction at thecurrent IP.

In addition, unstructured control flow is not currently supported inhardware for SIMD programs. Modern compilers produce unstructured SIMDprogram code which is traditionally converted back to structured programcode by the compiler, and some optimizations are lost due to convertingback to structured flow.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained from thefollowing detailed description in conjunction with the followingdrawings, in which:

FIG. 1 is a block diagram of an embodiment of a computer system with aprocessor having one or more processor cores and graphics processors;

FIG. 2 is a block diagram of one embodiment of a processor having one ormore processor cores, an integrated memory controller, and an integratedgraphics processor;

FIG. 3 is a block diagram of one embodiment of a graphics processorwhich may be a discreet graphics processing unit, or may be graphicsprocessor integrated with a plurality of processing cores;

FIG. 4 is a block diagram of an embodiment of a graphics-processingengine for a graphics processor;

FIG. 5 is a block diagram of another embodiment of a graphics processor;

FIG. 6 is a block diagram of thread execution logic including an arrayof processing elements;

FIG. 7 illustrates a graphics processor execution unit instructionformat according to an embodiment;

FIG. 8 is a block diagram of another embodiment of a graphics processorwhich includes a graphics pipeline, a media pipeline, a display engine,thread execution logic, and a render output pipeline;

FIG. 9A is a block diagram illustrating a graphics processor commandformat according to an embodiment;

FIG. 9B is a block diagram illustrating a graphics processor commandsequence according to an embodiment;

FIG. 10 illustrates exemplary graphics software architecture for a dataprocessing system according to an embodiment;

FIG. 11 illustrates one embodiment of a system architecture forperforming SIMD branch operations;

FIG. 12 illustrates a branch unit in accordance with one embodiment ofthe invention;

FIG. 13 illustrates a method for processing diverging control flowinstructions in accordance with one embodiment of the invention;

FIG. 14 illustrates a method for processing converging control flowinstructions in accordance with one embodiment of the invention;

FIG. 15 illustrates one embodiment of a branch unit with unstructuredcontrol flow logic;

FIG. 16 illustrates additional details of one embodiment of the branchunit;

FIG. 17 illustrates a method for processing unstructured divergingcontrol flow instructions in accordance with one embodiment of theinvention; and

FIG. 18 illustrates a method for processing unstructured convergingcontrol flow instructions in accordance with one embodiment of theinvention.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the embodiments of the invention described below. Itwill be apparent, however, to one skilled in the art that theembodiments of the invention may be practiced without some of thesespecific details. In other instances, well-known structures and devicesare shown in block diagram form to avoid obscuring the underlyingprinciples of the embodiments of the invention.

Exemplary Graphics Processor Architectures and Data Types

Overview—FIGS. 1-3

FIG. 1 is a block diagram of a data processing system 100, according toan embodiment. The data processing system 100 includes one or moreprocessors 102 and one or more graphics processors 108, and may be asingle processor desktop system, a multiprocessor workstation system, ora server system having a large number of processors 102 or processorcores 107. In on embodiment, the data processing system 100 is a systemon a chip integrated circuit (SOC) for use in mobile, handheld, orembedded devices.

An embodiment of the data processing system 100 can include, or beincorporated within a server-based gaming platform, a game console,including a game and media console, a mobile gaming console, a handheldgame console, or an online game console. In one embodiment, the dataprocessing system 100 is a mobile phone, smart phone, tablet computingdevice or mobile Internet device. The data processing system 100 canalso include, couple with, or be integrated within a wearable device,such as a smart watch wearable device, smart eyewear device, augmentedreality device, or virtual reality device. In one embodiment, the dataprocessing system 100 is a television or set top box device having oneor more processors 102 and a graphical interface generated by one ormore graphics processors 108.

The one or more processors 102 each include one or more processor cores107 to process instructions which, when executed, perform operations forsystem and user software. In one embodiment, each of the one or moreprocessor cores 107 is configured to process a specific instruction set109. The instruction set 109 may facilitate complex instruction setcomputing (CISC), reduced instruction set computing (RISC), or computingvia a very long instruction word (VLIW). Multiple processor cores 107may each process a different instruction set 109 which may includeinstructions to facilitate the emulation of other instruction sets. Aprocessor core 107 may also include other processing devices, such adigital signal processor (DSP).

In one embodiment, the processor 102 includes cache memory 104.Depending on the architecture, the processor 102 can have a singleinternal cache or multiple levels of internal cache. In one embodiment,the cache memory is shared among various components of the processor102. In one embodiment, the processor 102 also uses an external cache(e.g., a Level 3 (L3) cache or last level cache (LLC)) (not shown) whichmay be shared among the processor cores 107 using known cache coherencytechniques. A register file 106 is additionally included in theprocessor 102 which may include different types of registers for storingdifferent types of data (e.g., integer registers, floating pointregisters, status registers, and an instruction pointer register). Someregisters may be general-purpose registers, while other registers may bespecific to the design of the processor 102.

The processor 102 is coupled to a processor bus 110 to transmit datasignals between the processor 102 and other components in the system100. The system 100 uses an exemplary ‘hub’ system architecture,including a memory controller hub 116 and an input output (I/O)controller hub 130. The memory controller hub 116 facilitatescommunication between a memory device and other components of the system100, while the I/O controller hub (ICH) 130 provides connections to I/Odevices via a local I/O bus.

The memory device 120, can be a dynamic random access memory (DRAM)device, a static random access memory (SRAM) device, flash memorydevice, or some other memory device having suitable performance to serveas process memory. The memory 120 can store data 122 and instructions121 for use when the processor 102 executes a process. The memorycontroller hub 116 also couples with an optional external graphicsprocessor 112, which may communicate with the one or more graphicsprocessor 108 in the processor 102 to perform graphics and mediaoperations.

The ICH 130 enables peripherals to connect to the memory 120 andprocessor 102 via a high-speed I/O bus. The I/O peripherals include anaudio controller 146, a firmware interface 128, a wireless transceiver126 (e.g., Wi-Fi, Bluetooth), a data storage device 124 (e.g., hard diskdrive, flash memory, etc.), and a legacy I/O controller for couplinglegacy (e.g., Personal System 2 (PS/2)) devices to the system. One ormore Universal Serial Bus (USB) controllers 142 connect input devices,such as keyboard and mouse 144 combinations. A network controller 134may also couple to the ICH 130. In one embodiment, a high-performancenetwork controller (not shown) couples to the processor bus 110.

FIG. 2 is a block diagram of an embodiment of a processor 200 having oneor more processor cores 202A-N, an integrated memory controller 214, andan integrated graphics processor 208. The processor 200 can includeadditional cores up to and including additional core 202N represented bythe dashed lined boxes. Each of the cores 202A-N includes one or moreinternal cache units 204A-N. In one embodiment each core also has accessto one or more shared cached units 206.

The internal cache units 204A-N and shared cache units 206 represent acache memory hierarchy within the processor 200. The cache memoryhierarchy may include at least one level of instruction and data cachewithin each core and one or more levels of shared mid-level cache, suchas a level 2 (L2), level 3 (L3), level 4 (L4), or other levels of cache,where the highest level of cache before external memory is classified asthe last level cache (LLC). In one embodiment, cache coherency logicmaintains coherency between the various cache units 206 and 204A-N.

The processor 200 may also include a set of one or more bus controllerunits 216 and a system agent 210. The one or more bus controller unitsmanage a set of peripheral buses, such as one or more PeripheralComponent Interconnect buses (e.g., PCI, PCI Express). The system agent210 provides management functionality for the various processorcomponents. In one embodiment, the system agent 210 includes one or moreintegrated memory controllers 214 to manage access to various externalmemory devices (not shown).

In one embodiment, one or more of the cores 202A-N include support forsimultaneous multi-threading. In such embodiment, the system agent 210includes components for coordinating and operating cores 202A-N duringmulti-threaded processing. The system agent 210 may additionally includea power control unit (PCU), which includes logic and components toregulate the power state of the cores 202A-N and the graphics processor208.

The processor 200 additionally includes a graphics processor 208 toexecute graphics processing operations. In one embodiment, the graphicsprocessor 208 couples with the set of shared cache units 206, and thesystem agent unit 210, including the one or more integrated memorycontrollers 214. In one embodiment, a display controller 211 is coupledwith the graphics processor 208 to drive graphics processor output toone or more coupled displays. The display controller 211 may be separatemodule coupled with the graphics processor via at least oneinterconnect, or may be integrated within the graphics processor 208 orsystem agent 210.

In one embodiment a ring based interconnect unit 212 is used to couplethe internal components of the processor 200, however an alternativeinterconnect unit may be used, such as a point to point interconnect, aswitched interconnect, or other techniques, including techniques wellknown in the art. In one embodiment, the graphics processor 208 coupleswith the ring interconnect 212 via an I/O link 213.

The exemplary I/O link 213 represents at least one of multiple varietiesof I/O interconnects, including an on package I/O interconnect whichfacilitates communication between various processor components and ahigh-performance embedded memory module 218, such as an eDRAM module. Inone embodiment each of the cores 202-N and the graphics processor 208use the embedded memory modules 218 as shared last level cache.

In one embodiment cores 202A-N are homogenous cores executing the sameinstruction set architecture. In another embodiment, the cores 202A-Nare heterogeneous in terms of instruction set architecture (ISA), whereone or more of the cores 202A-N execute a first instruction set, whileat least one of the other cores executes a subset of the firstinstruction set or a different instruction set.

The processor 200 can be a part of or implemented on one or moresubstrates using any of a number of process technologies, for example,Complementary metal-oxide-semiconductor (CMOS), BipolarJunction/Complementary metal-oxide-semiconductor (BiCMOS) or N-typemetal-oxide-semiconductor logic (NMOS). Additionally, the processor 200can be implemented on one or more chips or as a system on a chip (SOC)integrated circuit having the illustrated components, in addition toother components.

FIG. 3 is a block diagram of one embodiment of a graphics processor 300which may be a discreet graphics processing unit, or may be graphicsprocessor integrated with a plurality of processing cores. In oneembodiment, the graphics processor is communicated with via a memorymapped I/O interface to registers on the graphics processor and viacommands placed into the processor memory. The graphics processor 300includes a memory interface 314 to access memory. The memory interface314 can be an interface to local memory, one or more internal caches,one or more shared external caches, and/or to system memory.

The graphics processor 300 also includes a display controller 302 todrive display output data to a display device 320. The displaycontroller 302 includes hardware for one or more overlay planes for thedisplay and composition of multiple layers of video or user interfaceelements. In one embodiment the graphics processor 300 includes a videocodec engine 306 to encode, decode, or transcode media to, from, orbetween one or more media encoding formats, including, but not limitedto Moving Picture Experts Group (MPEG) formats such as MPEG-2, AdvancedVideo Coding (AVC) formats such as H.264/MPEG-4 AVC, as well as theSociety of Motion Picture & Television Engineers (SMPTE) 421 M/VC-1, andJoint Photographic Experts Group (JPEG) formats such as JPEG, and MotionJPEG (MJPEG) formats.

In one embodiment, the graphics processor 300 includes a block imagetransfer (BLIT) engine 304 to perform two-dimensional (2D) rasterizeroperations including, for example, bit-boundary block transfers.However, in one embodiment, 2D graphics operations are performed usingone or more components of the graphics-processing engine (GPE) 310. Thegraphics-processing engine 310 is a compute engine for performinggraphics operations, including three-dimensional (3D) graphicsoperations and media operations.

The GPE 310 includes a 3D pipeline 312 for performing 3D operations,such as rendering three-dimensional images and scenes using processingfunctions that act upon 3D primitive shapes (e.g., rectangle, triangle,etc.). The 3D pipeline 312 includes programmable and fixed functionelements that perform various tasks within the element and/or spawnexecution threads to a 3D/Media sub-system 315. While the 3D pipeline312 can be used to perform media operations, an embodiment of the GPE310 also includes a media pipeline 316 that is specifically used toperform media operations, such as video post processing and imageenhancement.

In one embodiment, the media pipeline 316 includes fixed function orprogrammable logic units to perform one or more specialized mediaoperations, such as video decode acceleration, video de-interlacing, andvideo encode acceleration in place of, or on behalf of the video codecengine 306. In on embodiment, the media pipeline 316 additionallyincludes a thread spawning unit to spawn threads for execution on the3D/Media sub-system 315. The spawned threads perform computations forthe media operations on one or more graphics execution units included inthe 3D/Media sub-system.

The 3D/Media subsystem 315 includes logic for executing threads spawnedby the 3D pipeline 312 and media pipeline 316. In one embodiment, thepipelines send thread execution requests to the 3D/Media subsystem 315,which includes thread dispatch logic for arbitrating and dispatching thevarious requests to available thread execution resources. The executionresources include an array of graphics execution units to process the 3Dand media threads. In one embodiment, the 3D/Media subsystem 315includes one or more internal caches for thread instructions and data.In one embodiment, the subsystem also includes shared memory, includingregisters and addressable memory, to share data between threads and tostore output data.

3D/Media Processing—FIG. 4

FIG. 4 is a block diagram of an embodiment of a graphics processingengine 410 for a graphics processor. In one embodiment, the graphicsprocessing engine (GPE) 410 is a version of the GPE 310 shown in FIG. 3.The GPE 410 includes a 3D pipeline 412 and a media pipeline 416, each ofwhich can be either different from or similar to the implementations ofthe 3D pipeline 312 and the media pipeline 316 of FIG. 3.

In one embodiment, the GPE 410 couples with a command streamer 403,which provides a command stream to the GPE 3D and media pipelines 412,416. The command streamer 403 is coupled to memory, which can be systemmemory, or one or more of internal cache memory and shared cache memory.The command streamer 403 receives commands from the memory and sends thecommands to the 3D pipeline 412 and/or media pipeline 416. The 3D andmedia pipelines process the commands by performing operations via logicwithin the respective pipelines or by dispatching one or more executionthreads to the execution unit array 414. In one embodiment, theexecution unit array 414 is scalable, such that the array includes avariable number of execution units based on the target power andperformance level of the GPE 410.

A sampling engine 430 couples with memory (e.g., cache memory or systemmemory) and the execution unit array 414. In one embodiment, thesampling engine 430 provides a memory access mechanism for the scalableexecution unit array 414 that allows the execution array 414 to readgraphics and media data from memory. In one embodiment, the samplingengine 430 includes logic to perform specialized image samplingoperations for media.

The specialized media sampling logic in the sampling engine 430 includesa de-noise/de-interlace module 432, a motion estimation module 434, andan image scaling and filtering module 436. The de-noise/de-interlacemodule 432 includes logic to perform one or more of a de-noise or ade-interlace algorithm on decoded video data. The de-interlace logiccombines alternating fields of interlaced video content into a singlefame of video. The de-noise logic reduces or remove data noise fromvideo and image data. In one embodiment, the de-noise logic andde-interlace logic are motion adaptive and use spatial or temporalfiltering based on the amount of motion detected in the video data. Inone embodiment, the de-noise/de-interlace module 432 includes dedicatedmotion detection logic (e.g., within the motion estimation engine 434).

The motion estimation engine 434 provides hardware acceleration forvideo operations by performing video acceleration functions such asmotion vector estimation and prediction on video data. The motionestimation engine determines motion vectors that describe thetransformation of image data between successive video frames. In oneembodiment, a graphics processor media codec uses the video motionestimation engine 434 to perform operations on video at the macro-blocklevel that may otherwise be computationally intensive to perform using ageneral-purpose processor. In one embodiment, the motion estimationengine 434 is generally available to graphics processor components toassist with video decode and processing functions that are sensitive oradaptive to the direction or magnitude of the motion within video data.

The image scaling and filtering module 436 performs image-processingoperations to enhance the visual quality of generated images and video.In one embodiment, the scaling and filtering module 436 processes imageand video data during the sampling operation before providing the datato the execution unit array 414.

In one embodiment, the graphics processing engine 410 includes a dataport 444, which provides an additional mechanism for graphics subsystemsto access memory. The data port 444 facilitates memory access foroperations including render target writes, constant buffer reads,scratch memory space reads/writes, and media surface accesses. In oneembodiment, the data port 444 includes cache memory space to cacheaccesses to memory. The cache memory can be a single data cache orseparated into multiple caches for the multiple subsystems that accessmemory via the data port (e.g., a render buffer cache, a constant buffercache, etc.). In one embodiment, threads executing on an execution unitin the execution unit array 414 communicate with the data port byexchanging messages via a data distribution interconnect that coupleseach of the sub-systems of the graphics processing engine 410.

Execution Units—FIGS. 5-7

FIG. 5 is a block diagram of another embodiment of a graphics processor.In one embodiment, the graphics processor includes a ring interconnect502, a pipeline front-end 504, a media engine 537, and graphics cores580A-N. The ring interconnect 502 couples the graphics processor toother processing units, including other graphics processors or one ormore general-purpose processor cores. In one embodiment, the graphicsprocessor is one of many processors integrated within a multi-coreprocessing system.

The graphics processor receives batches of commands via the ringinterconnect 502. The incoming commands are interpreted by a commandstreamer 503 in the pipeline front-end 504. The graphics processorincludes scalable execution logic to perform 3D geometry processing andmedia processing via the graphics core(s) 580A-N. For 3D geometryprocessing commands, the command streamer 503 supplies the commands tothe geometry pipeline 536. For at least some media processing commands,the command streamer 503 supplies the commands to a video front end 534,which couples with a media engine 537. The media engine 537 includes avideo quality engine (VQE) 530 for video and image post processing and amulti-format encode/decode (MFX) 533 engine to providehardware-accelerated media data encode and decode. The geometry pipeline536 and media engine 537 each generate execution threads for the threadexecution resources provided by at least one graphics core 580A.

The graphics processor includes scalable thread execution resourcesfeaturing modular cores 580A-N (sometime referred to as core slices),each having multiple sub-cores 550A-N, 560A-N (sometimes referred to ascore sub-slices). The graphics processor can have any number of graphicscores 580A through 580N. In one embodiment, the graphics processorincludes a graphics core 580A having at least a first sub-core 550A anda second core sub-core 560A. In another embodiment, the graphicsprocessor is a low power processor with a single sub-core (e.g., 550A).In one embodiment, the graphics processor includes multiple graphicscores 580A-N, each including a set of first sub-cores 550A-N and a setof second sub-cores 560A-N. Each sub-core in the set of first sub-cores550A-N includes at least a first set of execution units 552A-N andmedia/texture samplers 554A-N. Each sub-core in the set of secondsub-cores 560A-N includes at least a second set of execution units562A-N and samplers 564A-N. In one embodiment, each sub-core 550A-N,560A-N shares a set of shared resources 570A-N. In one embodiment, theshared resources include shared cache memory and pixel operation logic.Other shared resources may also be included in the various embodimentsof the graphics processor.

FIG. 6 illustrates thread execution logic 600 including an array ofprocessing elements employed in one embodiment of a graphics processingengine. In one embodiment, the thread execution logic 600 includes apixel shader 602, a thread dispatcher 604, instruction cache 606, ascalable execution unit array including a plurality of execution units608A-N, a sampler 610, a data cache 612, and a data port 614. In oneembodiment the included components are interconnected via aninterconnect fabric that links to each of the components. The threadexecution logic 600 includes one or more connections to memory, such assystem memory or cache memory, through one or more of the instructioncache 606, the data port 614, the sampler 610, and the execution unitarray 608A-N. In one embodiment, each execution unit (e.g. 608A) is anindividual vector processor capable of executing multiple simultaneousthreads and processing multiple data elements in parallel for eachthread. The execution unit array 608A-N includes any number individualexecution units.

In one embodiment, the execution unit array 608A-N is primarily used toexecute “shader” programs. In one embodiment, the execution units in thearray 608A-N execute an instruction set that includes native support formany standard 3D graphics shader instructions, such that shader programsfrom graphics libraries (e.g., Direct 3D and OpenGL) are executed with aminimal translation. The execution units support vertex and geometryprocessing (e.g., vertex programs, geometry programs, vertex shaders),pixel processing (e.g., pixel shaders, fragment shaders) andgeneral-purpose processing (e.g., compute and media shaders).

Each execution unit in the execution unit array 608A-N operates onarrays of data elements. The number of data elements is the “executionsize,” or the number of channels for the instruction. An executionchannel is a logical unit of execution for data element access, masking,and flow control within instructions. The number of channels may beindependent of the number of physical ALUs or FPUs for a particulargraphics processor. The execution units 608A-N support integer andfloating-point data types.

The execution unit instruction set includes single instruction multipledata (SIMD) instructions. The various data elements can be stored as apacked data type in a register and the execution unit will process thevarious elements based on the data size of the elements. For example,when operating on a 256-bit wide vector, the 256 bits of the vector arestored in a register and the execution unit operates on the vector asfour separate 64-bit packed data elements (quad-word (QW) size dataelements), eight separate 32-bit packed data elements (double word (DW)size data elements), sixteen separate 16-bit packed data elements (word(W) size data elements), or thirty-two separate 8-bit data elements(byte (B) size data elements). However, different vector widths andregister sizes are possible.

One or more internal instruction caches (e.g., 606) are included in thethread execution logic 600 to cache thread instructions for theexecution units. In one embodiment, one or more data caches (e.g., 612)are included to cache thread data during thread execution. A sampler 610is included to provide texture sampling for 3D operations and mediasampling for media operations. In one embodiment, the sampler 610includes specialized texture or media sampling functionality to processtexture or media data during the sampling process before providing thesampled data to an execution unit.

During execution, the graphics and media pipelines send threadinitiation requests to the thread execution logic 600 via threadspawning and dispatch logic. The thread execution logic 600 includes alocal thread dispatcher 604 that arbitrates thread initiation requestsfrom the graphics and media pipelines and instantiates the requestedthreads on one or more execution units 608A-N. For example, the geometrypipeline (e.g., 536 of FIG. 5) dispatches vertex processing,tessellation, or geometry processing threads to the thread executionlogic 600. The thread dispatcher 604 can also process runtime threadspawning requests from the executing shader programs.

Once a group of geometric objects have been processed and rasterizedinto pixel data, the pixel shader 602 is invoked to further computeoutput information and cause results to be written to output surfaces(e.g., color buffers, depth buffers, stencil buffers, etc.). In oneembodiment, the pixel shader 602 calculates the values of the variousvertex attributes that are to be interpolated across the rasterizedobject. The pixel shader 602 then executes an API-supplied pixel shaderprogram. To execute the pixel shader program, the pixel shader 602dispatches threads to an execution unit (e.g., 608A) via the threaddispatcher 604. The pixel shader 602 uses texture sampling logic in thesampler 610 to access texture data in texture maps stored in memory.Arithmetic operations on the texture data and the input geometry datacompute pixel color data for each geometric fragment, or discards one ormore pixels from further processing.

In one embodiment, the data port 614 provides a memory access mechanismfor the thread execution logic 600 output processed data to memory forprocessing on a graphics processor output pipeline. In one embodiment,the data port 614 includes or couples to one or more cache memories(e.g., data cache 612) to cache data for memory access via the dataport.

FIG. 7 is a block diagram illustrating a graphics processor executionunit instruction format according to an embodiment. In one embodiment,the graphics processor execution units support an instruction set havinginstructions in multiple formats. The solid lined boxes illustrate thecomponents that are generally included in an execution unit instruction,while the dashed lines include components that are optional or that areonly included in a sub-set of the instructions. The instruction formatdescribed an illustrated are macro-instructions, in that they areinstructions supplied to the execution unit, as opposed tomicro-operations resulting from instruction decode once the instructionis processed.

In one embodiment, the graphics processor execution units nativelysupport instructions in a 128-bit format 710. A 64-bit compactedinstruction format 730 is available for some instructions based on theselected instruction, instruction options, and number of operands. Thenative 128-bit format 710 provides access to all instruction options,while some options and operations are restricted in the 64-bit format730. The native instructions available in the 64-bit format 730 variesby embodiment. In one embodiment, the instruction is compacted in partusing a set of index values in an index field 713. The execution unithardware references a set of compaction tables based on the index valuesand uses the compaction table outputs to reconstruct a nativeinstruction in the 128-bit format 710.

For each format, an instruction opcode 712 defines the operation thatthe execution unit is to perform. The execution units execute eachinstruction in parallel across the multiple data elements of eachoperand. For example, in response to an add instruction the executionunit performs a simultaneous add operation across each color channelrepresenting a texture element or picture element. By default, theexecution unit performs each instruction across all data channels of theoperands. An instruction control field 712 enables control over certainexecution options, such as channels selection (e.g., predication) anddata channel order (e.g., swizzle). For 128-bit instructions 710 anexec-size field 716 limits the number of data channels that will beexecuted in parallel. The exec-size field 716 is not available for usein the 64-bit compact instruction format 730.

Some execution unit instructions have up to three operands including twosource operands, src0 722, src1 722, and one destination 718. In oneembodiment, the execution units support dual destination instructions,where one of the destinations is implied. Data manipulation instructionscan have a third source operand (e.g., SRC2 724), where the instructionopcode JJ12 determines the number of source operands. An instruction'slast source operand can be an immediate (e.g., hard-coded) value passedwith the instruction.

In one embodiment instructions are grouped based on opcode bit-fields tosimplify Opcode decode 740. For an 8-bit opcode, bits 4, 5, and 6 allowthe execution unit to determine the type of opcode. The precise opcodegrouping shown is exemplary. In one embodiment, a move and logic opcodegroup 742 includes data movement and logic instructions (e.g., mov,cmp). The move and logic group 742 shares the five most significant bits(MSB), where move instructions are in the form of 0000xxxxb (e.g., 0x0x)and logic instructions are in the form of 0001 xxxxb (e.g., 0x01). Aflow control instruction group 744 (e.g., call, jmp) includesinstructions in the form of 0010xxxxb (e.g., 0x20). A miscellaneousinstruction group 746 includes a mix of instructions, includingsynchronization instructions (e.g., wait, send) in the form of 0011xxxxb(e.g., 0x30). A parallel math instruction group 748 includescomponent-wise arithmetic instructions (e.g., add, mul) in the form of0100xxxxb (e.g., 0x40). The parallel math group 748 performs thearithmetic operations in parallel across data channels. The vector mathgroup 750 includes arithmetic instructions (e.g., dp4) in the form of0101xxxxb (e.g., 0x50). The vector math group performs arithmetic suchas dot product calculations on vector operands.

Graphics Pipeline—FIG. 8

FIG. 8 is a block diagram of another embodiment of a graphics processorwhich includes a graphics pipeline 820, a media pipeline 830, a displayengine 840, thread execution logic 850, and a render output pipeline870. In one embodiment, the graphics processor is a graphics processorwithin a multi-core processing system that includes one or more generalpurpose processing cores. The graphics processor is controlled byregister writes to one or more control registers (not shown) or viacommands issued to the graphics processor via a ring interconnect 802.The ring interconnect 802 couples the graphics processor to otherprocessing components, such as other graphics processors orgeneral-purpose processors. Commands from the ring interconnect areinterpreted by a command streamer 803 which supplies instructions toindividual components of the graphics pipeline 820 or media pipeline830.

The command streamer 803 directs the operation of a vertex fetcher 805component that reads vertex data from memory and executesvertex-processing commands provided by the command streamer 803. Thevertex fetcher 805 provides vertex data to a vertex shader 807, whichperforms coordinate space transformation and lighting operations to eachvertex. The vertex fetcher 805 and vertex shader 807 executevertex-processing instructions by dispatching execution threads to theexecution units 852A, 852B via a thread dispatcher 831.

In one embodiment, the execution units 852A, 852B are an array of vectorprocessors having an instruction set for performing graphics and mediaoperations. The execution units 852A, 852B have an attached L1 cache 851that is specific for each array or shared between the arrays. The cachecan be configured as a data cache, an instruction cache, or a singlecache that is partitioned to contain data and instructions in differentpartitions.

In one embodiment, the graphics pipeline 820 includes tessellationcomponents to perform hardware-accelerated tessellation of 3D objects. Aprogrammable hull shader 811 configures the tessellation operations. Aprogrammable domain shader 817 provides back-end evaluation oftessellation output. A tessellator 813 operates at the direction of thehull shader 811 and contains special purpose logic to generate a set ofdetailed geometric objects based on a coarse geometric model that isprovided as input to the graphics pipeline 820. If tessellation is notused, the tessellation components 811, 813, 817 can be bypassed.

The complete geometric objects can be processed by a geometry shader 819via one or more threads dispatched to the execution units 852A, 852B, orcan proceed directly to the clipper 829. The geometry shader operates onentire geometric objects, rather than vertices or patches of vertices asin previous stages of the graphics pipeline. If the tessellation isdisabled the geometry shader 819 receives input from the vertex shader807. The geometry shader 819 is programmable by a geometry shaderprogram to perform geometry tessellation if the tessellation units aredisabled.

Prior to rasterization, vertex data is processed by a clipper 829, whichis either a fixed function clipper or a programmable clipper havingclipping and geometry shader functions. In one embodiment, a rasterizer873 in the render output pipeline 870 dispatches pixel shaders toconvert the geometric objects into their per pixel representations. Inone embodiment, pixel shader logic is included in the thread executionlogic 850.

The graphics engine has an interconnect bus, interconnect fabric, orsome other interconnect mechanism that allows data and message passingamongst the major components of the graphics engine. In one embodimentthe execution units 852A, 852B and associated cache(s) 851, texture andmedia sampler 854, and texture/sampler cache 858 interconnect via a dataport 856 to perform memory access and communicate with render outputpipeline components of the graphics engine. In one embodiment, thesampler 854, caches 851, 858 and execution units 852A, 852B each haveseparate memory access paths.

In one embodiment, the render output pipeline 870 contains a rasterizerand depth test component 873 that converts vertex-based objects intotheir associated pixel-based representation. In one embodiment, therasterizer logic includes a windower/masker unit to perform fixedfunction triangle and line rasterization. An associated render and depthbuffer caches 878, 879 are also available in one embodiment. A pixeloperations component 877 performs pixel-based operations on the data,though in some instances, pixel operations associated with 2D operations(e.g. bit block image transfers with blending) are performed by the 2Dengine 841, or substituted at display time by the display controller 843using overlay display planes. In one embodiment a shared L3 cache 875 isavailable to all graphics components, allowing the sharing of datawithout the use of main system memory.

The graphics processor media pipeline 830 includes a media engine 337and a video front end 834. In one embodiment, the video front end 834receives pipeline commands from the command streamer 803. However, inone embodiment the media pipeline 830 includes a separate commandstreamer. The video front-end 834 processes media commands beforesending the command to the media engine 837. In one embodiment, themedia engine includes thread spawning functionality to spawn threads fordispatch to the thread execution logic 850 via the thread dispatcher831.

In one embodiment, the graphics engine includes a display engine 840. Inone embodiment, the display engine 840 is external to the graphicsprocessor and couples with the graphics processor via the ringinterconnect 802, or some other interconnect bus or fabric. The displayengine 840 includes a 2D engine 841 and a display controller 843. Thedisplay engine 840 contains special purpose logic capable of operatingindependently of the 3D pipeline. The display controller 843 coupleswith a display device (not shown), which may be a system integrateddisplay device, as in a laptop computer, or an external display deviceattached via an display device connector.

The graphics pipeline 820 and media pipeline 830 are configurable toperform operations based on multiple graphics and media programminginterfaces and are not specific to any one application programminginterface (API). In one embodiment, driver software for the graphicsprocessor translates API calls that are specific to a particulargraphics or media library into commands that can be processed by thegraphics processor. In various embodiments, support is provided for theOpen Graphics Library (OpenGL) and Open Computing Language (OpenCL)supported by the Khronos Group, the Direct3D library from the MicrosoftCorporation, or, in one embodiment, both OpenGL and D3D. Support mayalso be provided for the Open Source Computer Vision Library (OpenCV). Afuture API with a compatible 3D pipeline would also be supported if amapping can be made from the pipeline of the future API to the pipelineof the graphics processor.

Graphics Pipeline Programming—FIG. 9A-B

FIG. 9A is a block diagram illustrating a graphics processor commandformat according to an embodiment and FIG. 9B is a block diagramillustrating a graphics processor command sequence according to anembodiment. The solid lined boxes in FIG. 9A illustrate the componentsthat are generally included in a graphics command while the dashed linesinclude components that are optional or that are only included in asub-set of the graphics commands. The exemplary graphics processorcommand format 900 of FIG. 9A includes data fields to identify a targetclient 902 of the command, a command operation code (opcode) 904, andthe relevant data 906 for the command. A sub-opcode 905 and a commandsize 908 are also included in some commands.

The client 902 specifies the client unit of the graphics device thatprocesses the command data. In one embodiment, a graphics processorcommand parser examines the client field of each command to conditionthe further processing of the command and route the command data to theappropriate client unit. In one embodiment, the graphics processorclient units include a memory interface unit, a render unit, a 2D unit,a 3D unit, and a media unit. Each client unit has a correspondingprocessing pipeline that processes the commands. Once the command isreceived by the client unit, the client unit reads the opcode 904 and,if present, sub-opcode 905 to determine the operation to perform. Theclient unit performs the command using information in the data 906 fieldof the command. For some commands an explicit command size 908 isexpected to specify the size of the command. In one embodiment, thecommand parser automatically determines the size of at least some of thecommands based on the command opcode. In one embodiment commands arealigned via multiples of a double word.

The flow chart in FIG. 9B shows a sample command sequence 910. In oneembodiment, software or firmware of a data processing system thatfeatures an embodiment of the graphics processor uses a version of thecommand sequence shown to set up, execute, and terminate a set ofgraphics operations. A sample command sequence is shown and describedfor exemplary purposes, however embodiments are not limited to thesecommands or to this command sequence. Moreover, the commands may beissued as batch of commands in a command sequence, such that thegraphics processor will process the sequence of commands in an at leastpartially concurrent manner.

The sample command sequence 910 may begin with a pipeline flush command912 to cause any active graphics pipeline to complete the currentlypending commands for the pipeline. In one embodiment, the 3D pipeline922 and the media pipeline 924 do not operate concurrently. The pipelineflush is performed to cause the active graphics pipeline to complete anypending commands. In response to a pipeline flush, the command parserfor the graphics processor will pause command processing until theactive drawing engines complete pending operations and the relevant readcaches are invalidated. Optionally, any data in the render cache that ismarked ‘dirty’ can be flushed to memory. A pipeline flush command 912can be used for pipeline synchronization or before placing the graphicsprocessor into a low power state.

A pipeline select command 913 is used when a command sequence requiresthe graphics processor to explicitly switch between pipelines. Apipeline select command 913 is required only once within an executioncontext before issuing pipeline commands unless the context is to issuecommands for both pipelines. In one embodiment, a pipeline flush commandis 912 is required immediately before a pipeline switch via the pipelineselect command 913.

A pipeline control command 914 configures a graphics pipeline foroperation and is used to program the 3D pipeline 922 and the mediapipeline 924. The pipeline control command 914 configures the pipelinestate for the active pipeline. In one embodiment, the pipeline controlcommand 914 is used for pipeline synchronization and to clear data fromone or more cache memories within the active pipeline before processinga batch of commands.

Return buffer state commands 916 are used to configure a set of returnbuffers for the respective pipelines to write data. Some pipelineoperations require the allocation, selection, or configuration of one ormore return buffers into which the operations write intermediate dataduring processing. The graphics processor also uses one or more returnbuffers to store output data and to perform cross thread communication.The return buffer state 916 includes selecting the size and number ofreturn buffers to use for a set of pipeline operations.

The remaining commands in the command sequence differ based on theactive pipeline for operations. Based on a pipeline determination 920,the command sequence is tailored to the 3D pipeline 922 beginning withthe 3D pipeline state 930, or the media pipeline 924 beginning at themedia pipeline state 940.

The commands for the 3D pipeline state 930 include 3D state settingcommands for vertex buffer state, vertex element state, constant colorstate, depth buffer state, and other state variables that are to beconfigured before 3D primitive commands are processed. The values ofthese commands are determined at least in part based the particular 3DAPI in use. 3D pipeline state 930 commands are also able to selectivelydisable or bypass certain pipeline elements if those elements will notbe used.

The 3D primitive 932 command is used to submit 3D primitives to beprocessed by the 3D pipeline. Commands and associated parameters thatare passed to the graphics processor via the 3D primitive 932 commandare forwarded to the vertex fetch function in the graphics pipeline. Thevertex fetch function uses the 3D primitive 932 command data to generatevertex data structures. The vertex data structures are stored in one ormore return buffers. The 3D primitive 932 command is used to performvertex operations on 3D primitives via vertex shaders. To process vertexshaders, the 3D pipeline 922 dispatches shader execution threads tographics processor execution units.

The 3D pipeline 922 is triggered via an execute 934 command or event. Inone embodiment a register write triggers command execution. In oneembodiment execution is triggered via a ‘go’ or ‘kick’ command in thecommand sequence. In one embodiment command execution is triggered usinga pipeline synchronization command to flush the command sequence throughthe graphics pipeline. The 3D pipeline will perform geometry processingfor the 3D primitives. Once operations are complete, the resultinggeometric objects are rasterized and the pixel engine colors theresulting pixels. Additional commands to control pixel shading and pixelback end operations may also be included for those operations.

The sample command sequence 910 follows the media pipeline 924 path whenperforming media operations. In general, the specific use and manner ofprogramming for the media pipeline 924 depends on the media or computeoperations to be performed. Specific media decode operations may beoffloaded to the media pipeline during media decode. The media pipelinecan also be bypassed and media decode can be performed in whole or inpart using resources provided by one or more general purpose processingcores. In one embodiment, the media pipeline also includes elements forgeneral-purpose graphics processor unit (GPGPU) operations, where thegraphics processor is used to perform SIMD vector operations usingcomputational shader programs that are not explicitly related to therendering of graphics primitives.

The media pipeline 924 is configured in a similar manner as the 3Dpipeline 922. A set of media pipeline state commands 940 are dispatchedor placed into in a command queue before the media object commands 942.The media pipeline state commands 940 include data to configure themedia pipeline elements that will be used to process the media objects.This includes data to configure the video decode and video encode logicwithin the media pipeline, such as encode or decode format. The mediapipeline state commands 940 also support the use one or more pointers to“indirect” state elements that contain a batch of state settings.

Media object commands 942 supply pointers to media objects forprocessing by the media pipeline. The media objects include memorybuffers containing video data to be processed. In one embodiment, allmedia pipeline state must be valid before issuing a media object command942. Once the pipeline state is configured and media object commands 942are queued, the media pipeline 924 is triggered via an execute 934command or an equivalent execute event (e.g., register write). Outputfrom the media pipeline 924 may then be post processed by operationsprovided by the 3D pipeline 922 or the media pipeline 924. In oneembodiment, GPGPU operations are configured and executed in a similarmanner as media operations.

Graphics Software Architecture—FIG. 10

FIG. 10 illustrates exemplary graphics software architecture for a dataprocessing system according to an embodiment. The software architectureincludes a 3D graphics application 1010, an operating system 1020, andat least one processor 1030. The processor 1030 includes a graphicsprocessor 1032 and one or more general-purpose processor core(s) 1034.The graphics application 1010 and operating system 1020 each execute inthe system memory 1050 of the data processing system.

In one embodiment, the 3D graphics application 1010 contains one or moreshader programs including shader instructions 1012. The shader languageinstructions may be in a high-level shader language, such as the HighLevel Shader Language (HLSL) or the OpenGL Shader Language (GLSL). Theapplication also includes executable instructions 1014 in a machinelanguage suitable for execution by the general-purpose processor core1034. The application also includes graphics objects 1016 defined byvertex data.

The operating system 1020 may be a Microsoft® Windows® operating systemfrom the Microsoft Corporation, a proprietary UNIX-like operatingsystem, or an open source UNIX-like operating system using a variant ofthe Linux kernel. When the Direct3D API is in use, the operating system1020 uses a front-end shader compiler 1024 to compile any shaderinstructions 1012 in HLSL into a lower-level shader language. Thecompilation may be a just-in-time compilation or the application canperform share pre-compilation. In one embodiment, high-level shaders arecompiled into low-level shaders during the compilation of the 3Dgraphics application 1010.

The user mode graphics driver 1026 may contain a back-end shadercompiler 1027 to convert the shader instructions 1012 into a hardwarespecific representation. When the OpenGL API is in use, shaderinstructions 1012 in the GLSL high-level language are passed to a usermode graphics driver 1026 for compilation. The user mode graphics driveruses operating system kernel mode functions 1028 to communicate with akernel mode graphics driver 1029. The kernel mode graphics driver 1029communicates with the graphics processor 1032 to dispatch commands andinstructions.

To the extent various operations or functions are described herein, theycan be described or defined as hardware circuitry, software code,instructions, configuration, and/or data. The content can be embodied inhardware logic, or as directly executable software (“object” or“executable” form), source code, high level shader code designed forexecution on a graphics engine, or low level assembly language code inan instruction set for a specific processor or graphics core. Thesoftware content of the embodiments described herein can be provided viaan article of manufacture with the content stored thereon, or via amethod of operating a communication interface to send data via thecommunication interface.

A non-transitory machine readable storage medium can cause a machine toperform the functions or operations described, and includes anymechanism that stores information in a form accessible by a machine(e.g., computing device, electronic system, etc.), such asrecordable/non-recordable media (e.g., read only memory (ROM), randomaccess memory (RAM), magnetic disk storage media, optical storage media,flash memory devices, etc.). A communication interface includes anymechanism that interfaces to any of a hardwired, wireless, optical,etc., medium to communicate to another device, such as a memory businterface, a processor bus interface, an Internet connection, a diskcontroller, etc. The communication interface is configured by providingconfiguration parameters or sending signals to prepare the communicationinterface to provide a data signal describing the software content. Thecommunication interface can be accessed via one or more commands orsignals sent to the communication interface.

Various components described can be a means for performing theoperations or functions described. Each component described hereinincludes software, hardware, or a combination of these. The componentscan be implemented as software modules, hardware modules,special-purpose hardware (e.g., application specific hardware,application specific integrated circuits (ASICs), digital signalprocessors (DSPs), etc.), embedded controllers, hardwired circuitry,etc. Besides what is described herein, various modifications can be madeto the disclosed embodiments and implementations of the inventionwithout departing from their scope. Therefore, the illustrations andexamples herein should be construed in an illustrative, and not arestrictive sense. The scope of the invention should be measured solelyby reference to the claims that follow.

Apparatus and Method for SIMD Structured Branching

One embodiment of the invention performs SIMD control flow in hardware,thereby increasing performance and reducing power. In particular, a SIMDprocessor functional unit is described that can be used in SIMD threadswhere channels diverge and re-converge using structured flow. Oneexample of channel divergence is an IF/ELSE/ENDIF sequence where, if the“IF” condition is met, the instruction stream diverges to a firstinstruction sequence, but if the “IF” condition is not met, theinstruction stream diverges to a second instruction sequence (specifiedby the “ELSE” condition). Channel convergence occurs in response to theENDIF statement (e.g., when all divergent instructions required by the“IF” and/or “ELSE” statements have been completed).

In one embodiment of the invention, channel divergence and convergenceare detected by a new branch unit (e.g., via new instructions) whichresponsively enables and disables each channel. The branch unit may beshared across all threads in an execution unit (EU) and may compute theresult of all structured control flow instructions for all channels inthe processor.

One embodiment of the branch unit computes a channel mask, one bit perchannel, to enable or disable channels during structured control flowprocessing. This removes the necessity to compare channel IPs for everyinstruction. In one embodiment, the channel mask is used as-is for allnon-control flow instructions, which are processed by another unit.

In one embodiment, the channel enable mask is generated from a counterper channel which indicates if the channel is disabled, and if so, bywhat control flow instruction (i.e., IF/ELSE/ENDIF, CALL/RETURN, etc).This saves gates when compared with existing per-channel IP solution andsaves compute cycles per instruction. The embodiments of the inventionmay be implemented with any structured control flow instructionssupported by a standard (e.g., c/c++). This includes, by way of exampleand not limitation: IF/ELSE/ENDIF, DO/BREAK/CONTINUE/WHILE, CALL/RETURN,HALT, BRANCH, and may work with any other structured control flowinstructions.

Recursion is also supported in one embodiment using a counter thatdifferentiates occurrences of control flow instructions. In particular,in one embodiment, a “Control Instruction Count” is maintained for eachbranching type (IF/ELSE/ENDIF, DO/BREAK/CONTINUE/WHILE, CALL/RETURN,etc), which provides a count of the current level of nesting of thatbranching type. For example, a value of 2 for an IF branching type meanstwo nested IF statements. In one embodiment, the Control InstructionCount is zero if the current IP is outside of any branching conditionalblocks.

One embodiment also maintains a “Per Channel Count” to determine if achannel is enabled. If the Per Channel Count is 0, the channel isenabled. In one embodiment, when a channel is disabled, itscorresponding count contains the value of the Control Instruction Countat which the channel was disabled along with an encoded ID representingthe branching type. (e.g., IF, 2 for an IF branching type with 2 nestedIF statements).

In one embodiment, an n-channel entry stack is used to store the address(IP) of converging instructions (e.g., for an IF instruction, the IP ofthe ENDIF instruction is stored). A Top of Stack pointer is maintainedfor each branching type to identify the IP for the converginginstruction within the stack.

In addition, in one embodiment, each instruction contains a predicatemask. Each bit of the predicate mask represents one channel, and is usedto determine if the channel takes the conditional or not. (e.g., for IF,a channel can either enter the IF block, or jump to the ELSE block). Inone embodiment, only enabled channels are evaluated for execution.

FIG. 11 illustrates an architecture for an execution unit 1100 in whicha branch unit 1101 is implemented in accordance with one embodiment ofthe invention. A plurality of channels 0-2 execute instructions storedin an instruction memory 1105 which may be an instruction buffer,instruction cache or any other memory suitable for storing instructionsprior to execution. While 3 channels are illustrated in FIG. 11, theunderlying principles of the invention may be implemented with anynumber of channels. An instruction pointer 1110-1112 associated witheach channel 0-2, respectively, identifies the next instruction to beexecuted over its respective channel. Instruction dispatch logic1120-1122 then dispatches the instructions for execution on a pluralityof functional units 1130, which may include, for example, vectorfunctional units, scalar functional units, and logical functional units(and/or any other type of functional unit designed to execute theinstructions).

The execution unit 1100 shown in FIG. 11 may be implemented within agraphics processing unit architecture such as described above (see,e.g., execution units 608 shown in FIG. 6). However, the underlyingprinciples of the invention are not limited to a graphics processingimplementation.

As mentioned above, channel divergence and convergence are detected bythe branch unit 1101 which responsively enables and disables eachchannel. The branch unit 1101 may be shared across all threads in the EU1100 and may compute the result of all structured control flowinstructions for all channels in the processor. In one embodiment, eachthread is executed on a different channel 0-2.

FIG. 12 illustrates additional details associated with one embodiment ofthe branch unit 1101. As illustrated, this embodiment of the branch unit1101 computes a channel enable mask 1250, one bit per channel, to enableor disable channels during structured control flow processing (therebyremoving the necessity to compare channel IPs for every instruction). Inone embodiment, the channel mask 1250 is used as-is for all non-controlflow instructions, which are processed by another unit.

In one embodiment, the channel enable mask 1250 is generated from a setof per-channel counters 1200-1202 which indicate whether the channel isdisabled, and if so, by what control flow instruction (i.e.,IF/ELSE/ENDIF, CALL/RETURN, etc). A set of control instruction counters1211 differentiates occurrences of control flow instructions with a“control instruction count” maintained for each branching type(IF/ELSE/ENDIF, DO/BREAK/CONTINUE/WHILE, CALL/RETURN, etc). In oneembodiment, the control instruction count indicates the current level ofnesting of that branching type. In the example shown in FIG. 12, a valueof 2 is shown for an IF branching type, meaning two nested IFstatements. Values of 1 are shown for CALL/RETURN andDO/BREAK/CONTINUE/WHILE instructions indicating 1 level of nesting. Inone embodiment, the control instruction count is zero if the current IPis outside of any branching conditional blocks.

In one embodiment, if the per channel count is 0 in a per-channelcounter 1200-1202, then the corresponding channel is enabled, asindicated by a 0 in the channel mask 1250. In one embodiment, when achannel is disabled, its corresponding count in its per-channel counter1200-1202 comprises the control instruction count at which the channelwas disabled along with the encoded ID representing the branching type(provided by the control instruction counters 1211).

FIG. 12 also illustrates an n-channel entry stack 1260 used to store theaddress (IP) of converging instructions (e.g., for an IF instruction,the IP of the ENDIF instruction is stored). Top of stack pointers 1261are maintained for each branching type to identify the IP 1250-1254 forthe converging instruction within the stack 1260. In one embodiment, theIP of the converging instruction is added to the stack when executingthe corresponding diverging instruction.

As mentioned, in one embodiment, each instruction contains a predicatemask where each bit represents one channel, and is used to determine ifthe channel takes the conditional or not (e.g., for IF, a channel caneither enter the IF block, or jump to the ELSE block). In oneembodiment, only enabled channels are evaluated for execution.

A method for executing diverging instructions is set forth in FIG. 13.The method may be implemented within the context of the system shown inFIGS. 11-12 but is not limited to any particular system architecture.

Starting at 1301, for each channel n, a determination is made at 1302 asto whether the per channel count is 0. As mentioned, in one embodiment,a channel count of 0 indicates that the corresponding channel isenabled, determined at 1303. If the per channel count is non-zero, thecorresponding channel is not enabled, determined at 1304. The aboveprocess is performed for each channel n.

At 1305 a tag for the converging IP is set equal to the current IP+theconverging offset (i.e., the distance from the current IP to theconverging IP). At 1306, a determination is made as to whether allenabled channels have the same predicate value associated therewith(e.g., whether the predicate mask of the instructions are the same). Ifnot, then at 1307, the control instruction count is incremented for thecurrent control flow instruction type (e.g., the IF count is incrementedwithin the corresponding control instruction counter 1211).

At 1308, for each channel n, a determination is made as to whether thechannel is enabled at 1309. If so, then at 1310 a determination is madeas to whether the channel is enabled for the current instruction basedon the predicate enable field. If not, then the per channel count is setequal to the control instruction count for the control flow instructionat 1311 (thereby disabling the channel). The above process is repeatedfor each channel n. At 1312, the next instruction pointer is incremented(e.g., the next instruction pointer is set equal to the currentinstruction pointer+1).

Turning back to decision block 1306, if all enabled channels have thesame predicate value associated therewith (i.e., the predicate mask ofthe instructions are the same), then at 1313, a determination is made asto whether all active channels take the current condition (e.g., whetherall channels can process the control flow instruction). If so, then at1315 the next instruction pointer is set equal to the currentinstruction pointer added to the jump offset (i.e., the IP location ofthe diverging instruction). If not, then at 1314 the next instructionpointer is set equal to the current instruction pointer added to theconverging offset (i.e., the location of the converging instruction).

As a result of the foregoing method, if the predicate is uniform for allchannels, then the per channel count is not updated, and only theExecution IP is updated. If the predicate mask is not uniform for allchannels, this is called divergence and, in this case, some channelstake the condition and some do not. In this instance, the controlinstruction count is incremented, and the per channel count of eachchannel which does not take the condition is updated with the controlinstruction count of the current branching type. The Tag is pushed ontothe stack (to store the IP of the converging instruction), and the topof stack pointer is updated.

FIG. 14 illustrates one embodiment of a method for processing convergingcontrol flow instructions. The method may be implemented within thecontext of the processor architecture shown in FIGS. 11-12 but is notlimited to any particular architecture.

At 1401, a tag is set equal to the current instruction pointer. At 1402,if the tag is equal to the instruction pointer at the top of the stack(e.g., stack 1260), then the process moves to 1403. For each channel n,if the per channel count is equal to the control instruction count,determined at 1404, then the per channel count is set equal to 0 at 1405(enabling the channel).

Once the process completes for each channel, at 1406 a determination ismade as to whether any channel count is equal to the control instructioncount. If not, then at 1407, the control instruction count isdecremented and the instruction pointer is popped from the stack. At1408 a determination is made as to whether any channel is enabled. Ifso, then at 1409 the instruction pointer is incremented (i.e., the nextinstruction pointer is set to the current instruction pointer+1). Ifnot, then at 1410, the next instruction pointer is set equal to thecurrent instruction pointer added to the jump offset (i.e., to arrive atthe IP for the divergent control flow instruction).

As a result of the foregoing method, the current IP is compared to theTop of Stack IP. If those values match, then each per channel count iscompared to the branching type's control instruction count, for channelswith an enabled predicate mask bit. If these values match, the perchannel count is set to 0 (indicating an active channel). If allchannels are enabled corresponding to a particular control instructioncount, then the count is decremented and the stack is popped. The IP isincremented and the program continues as normal.

Apparatus and Method for Unstructured Control Flow for a SIMD ExecutionEngine

As mentioned above, unstructured control flow is not currently supportedin hardware for SIMD programs. Modern compilers produce unstructuredSIMD program code which is traditionally converted back to structuredprogram code by the compiler, and some optimizations are lost due toconverting back to structured flow.

To address this limitation, one embodiment of the invention supportsunstructured control flow instructions for SIMD programs in hardware,thereby maintaining the compiler optimizations which improvesperformance and reduces power. In particular, a branch unit may beequipped with logic to implement both the structured branchingembodiments (described above) and unstructured branching embodiments(described below). It should be noted, however, that the structuredbranching techniques described above are not required for implementingthe unstructured branching techniques described below (i.e., theunstructured branching techniques may be implemented in a branch unitwith or without the structured branching techniques).

As illustrated in FIG. 15, in one embodiment, the branch unit 1501includes unstructured control flow logic 1500 to support newunstructured control flow instructions. The unstructured control flowlogic 1500 may be used in SIMD threads where channels diverge andre-converge using unstructured control flow. Channel divergence andconvergence is may be detected from new instructions and the branch unit1501 manages enables for each channel as described below.

As in the structured control flow embodiments, a plurality of channels0-2 execute instructions stored in an instruction memory 1105 which maybe an instruction buffer, instruction cache or any other memory suitablefor storing instructions prior to execution. While three channels areillustrated in FIG. 15, the underlying principles of the invention maybe implemented with any number of channels. An instruction pointer1110-1112 associated with each channel 0-2, respectively, identifies thenext instruction to be executed over its respective channel. Instructiondispatch logic 1120-1122 then dispatches the instructions for executionon a plurality of functional units 1130, which may include, for example,vector functional units, scalar functional units, and logical functionalunits (and/or any other type of functional unit designed to execute theinstructions). The execution unit 1100 shown in FIG. 15 may beimplemented within a graphics processing unit architecture such asdescribed above (see, e.g., execution units 608 shown in FIG. 6).However, the underlying principles of the invention are not limited to agraphics processing implementation.

As mentioned above, channel divergence and convergence are detected bythe branch unit 1101 which responsively enables and disables eachchannel. The branch unit 1101 may be shared across all threads in the EU1100 and may compute the result of all unstructured control flowinstructions for all channels in the processor. In one embodiment, eachthread is executed on a different channel 0-2.

FIG. 16 provides additional details for implementing unstructuredcontrol flow including a channel mask 1250 to enable and disablechannels. The channel mask 1250 may be computed for both structured andunstructured control flow instructions by the branch unit 1501 and maybe used as-is for all non-control flow instructions, which are processedby another unit. In one embodiment, two new instructions are added forunstructured control flow, GOTO and JOIN, and a new counter 1605 isadded to process these new instructions. In one embodiment, GOTO causesa jump to a specified location in the instruction stream and JOINreturns to a specified location in the instruction stream (e.g., bothspecified via an address/instruction pointer).

In the embodiments described above related to SIMD structured branching,the branch unit 1501 has defined a Control Instruction Count perbranching type (e.g., IF/THEN/ENDIF, BREAK/WHILE/CONTINUE, CALL/RETURN,etc), which is a count of the current level of nesting of that branchingtype. In one embodiment, this mechanism is modified to handleunstructured instructions. Instead of using a count as the uniqueidentifier, this embodiment uses the address of the IP tag in the localmemory. While the unstructured instructions used a stack structure 1260shown in FIG. 12 (top of stack being the last address pushed onto thestack), the new unstructured instructions use a sorted linked-liststructure 1660 to store IP tags 1650-1654 in the same memory space asshown in FIG. 16. One difference with the stack structure 1260 is thatin the sorted linked-list structure 1660 the unstructured instruction IPtags 1650-1654 are sorted by hardware (while in the stack structure thestructured IPs are sorted by definition).

The existing Per Channel Count maintained in per channel counters1200-1202 is used to determine if a channel is enabled. If the PerChannel Count is 0, the channel is enabled. In one embodiment, when achannel is disabled by an unstructured instruction its correspondingcount is updated to contain the address of the IP tag 1650-1654 at whichthe channel was disabled along with an encoded ID representing thebranching type (e.g., “GOTO2” to identify the GOTO instruction and IPtag at entry #2 in the linked list memory 1660 structure 1660).

As in the prior embodiments, each instruction may include a predicatemask. Each bit in the predicate mask represents one channel, and is usedby the unstructured control flow logic 1500 of the branch unit 1501 todetermine if the channel takes the conditional or not. Only enabledchannels are evaluated for execution.

One embodiment of a method for executing unstructured diverginginstructions is set forth in FIG. 17. The method may be implementedwithin the context of the system shown in FIGS. 15-16 but is not limitedto any particular system architecture.

Starting at 1701, for each channel n, a determination is made at 1702 asto whether the per channel count is 0. As mentioned, in one embodiment,a channel count of 0 indicates that the corresponding channel isenabled, determined at 1703. If the per channel count is non-zero, thecorresponding channel is not enabled, determined at 1704. The aboveprocess is performed for each channel n.

At 1705 a tag for the converging IP is set equal to the current IP+theconverging offset (i.e., the distance from the current IP to the IP ofthe converging instruction). At 1706, a determination is made as towhether all enabled channels have the same predicate value associatedtherewith (e.g., whether the predicate mask of the instructions are thesame). If not, then at 1707, the sorted linked list memory structure(e.g., 1660 in FIG. 16) is searched to determine whether the IP tag isfound in the memory. If not, then at 1708, the tag is inserted into thememory and the tag address is saved (e.g., into a temporary storage suchas counter 1606 accessible by the unstructured control flow logic 1500).If the IP tag is found in the memory, then at 1709 the tag address issaved.

Starting at 1710, for each channel n, a determination is made as towhether the channel is enabled at 1711. If so, then at 1712 adetermination is made as to whether the channel is enabled for thecurrent instruction based on the predicate enable field of theinstruction. If not, then the per channel count is set equal to the tagaddress (identifying the IP tag within the memory 1660). In oneembodiment, the tag address is stored in the corresponding Per ChannelCounter 1200-1202 for the current channel n (thereby disabling thechannel). The above process is repeated for each channel n. At 1714, thenext instruction pointer is set by adding the jump offset to the currentinstruction pointer (i.e., to arrive at the IP address of the diverginginstruction).

Turning back to decision block 1706, if all enabled channels have thesame predicate value associated therewith (i.e., the predicate mask ofthe instructions are the same), then at 1715, a determination is made asto whether all active channels take the current condition (e.g., whetherall channels can process the control flow instruction). If so, then at1717 the next instruction pointer is set equal to the currentinstruction pointer added to the jump offset (i.e., the IP location ofthe diverging instruction). If not, then at 1716 the instruction pointeris incremented, i.e., the next instruction pointer is set equal to thecurrent instruction pointer+1 (i.e., the diverging instruction is nottaken).

As a result of the foregoing method, if the predicate is uniform for allchannels, then the per channel count is not updated, and only theExecution IP is updated. If the predicate mask is not uniform for allchannels, this is called divergence and, in this case, some channelstake the condition and some do not. In this instance, the controlinstruction count is incremented, and the per channel count of eachchannel which does not take the condition is updated with the memoryaddress of the converging IP. The IP Tag 1650-1654 is inserted intomemory 1660.

FIG. 18 illustrates one embodiment of a method for processingunstructured converging instructions. The method may be implementedwithin the context of the processor architecture shown in FIGS. 15-16but is not limited to any particular architecture.

At 1801, the tag is set equal to the current instruction pointer. At1802, the memory containing IP tags (e.g., sorted linked-list memory1660) is searched using the tag. If a match is found, then the tagaddress is saved at 1803 (e.g., into a temporary storage such as acounter 1606 accessible by the unstructured control flow logic). At1804, for each channel n, if the predicate enable bit is set (determinedat 1805) and the per channel count is equal to the tag address(determined at 1806), then the per channel count is set equal to 0 at1807 (thereby enabling the channel).

Once the process completes for each channel, at 1808 a determination ismade as to whether any channel count is equal to the tag address. Ifnot, then at 1809, the IP tag is deleted from memory. At 1810 adetermination is made as to whether any channel is enabled. If so, thenat 1811 the instruction pointer is incremented (i.e., the nextinstruction pointer is set to the current instruction pointer+1). Ifnot, then at 1812 the next instruction pointer is set equal to thecurrent instruction pointer added to the jump offset (i.e., to arrive atthe IP for the next unstructured instruction).

As a result of the foregoing method, the current IP is searched for inmemory. If it is found, then each Per Channel Count is compared to thememory address for channels with an enabled predicate mask bit. If thesevalues match, the Per Channel Count is set to 0 (indicating an activechannel). If all channels are enabled corresponding to a particularControl Instruction Count, then the Tag is removed from memory.

Embodiments of the invention may include various steps, which have beendescribed above. The steps may be embodied in machine-executableinstructions which may be used to cause a general-purpose orspecial-purpose processor to perform the steps. Alternatively, thesesteps may be performed by specific hardware components that containhardwired logic for performing the steps, or by any combination ofprogrammed computer components and custom hardware components.

As described herein, instructions may refer to specific configurationsof hardware such as application specific integrated circuits (ASICs)configured to perform certain operations or having a predeterminedfunctionality or software instructions stored in memory embodied in anon-transitory computer readable medium. Thus, the techniques shown inthe figures can be implemented using code and data stored and executedon one or more electronic devices (e.g., an end station, a networkelement, etc.). Such electronic devices store and communicate(internally and/or with other electronic devices over a network) codeand data using computer machine-readable media, such as non-transitorycomputer machine-readable storage media (e.g., magnetic disks; opticaldisks; random access memory; read only memory; flash memory devices;phase-change memory) and transitory computer machine-readablecommunication media (e.g., electrical, optical, acoustical or other formof propagated signals—such as carrier waves, infrared signals, digitalsignals, etc.). In addition, such electronic devices typically include aset of one or more processors coupled to one or more other components,such as one or more storage devices (non-transitory machine-readablestorage media), user input/output devices (e.g., a keyboard, atouchscreen, and/or a display), and network connections. The coupling ofthe set of processors and other components is typically through one ormore busses and bridges (also termed as bus controllers). The storagedevice and signals carrying the network traffic respectively representone or more machine-readable storage media and machine-readablecommunication media. Thus, the storage device of a given electronicdevice typically stores code and/or data for execution on the set of oneor more processors of that electronic device. Of course, one or moreparts of an embodiment of the invention may be implemented usingdifferent combinations of software, firmware, and/or hardware.Throughout this detailed description, for the purposes of explanation,numerous specific details were set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the invention may be practiced without someof these specific details. In certain instances, well known structuresand functions were not described in elaborate detail in order to avoidobscuring the subject matter of the present invention. Accordingly, thescope and spirit of the invention should be judged in terms of theclaims which follow.

What is claimed is:
 1. A processor comprising: an execution unit havinga plurality of channels to execute instructions; and a branch unit toprocess unstructured control flow instructions and to maintain a perchannel count value for each channel, the branch unit to storeinstruction pointer tags for the unstructured control flow instructionsin a memory and identify the instruction pointer tags using tagaddresses, the branch unit to further enable and disable the channelsbased at least on the per channel count value, and wherein the branchunit is to generate a channel enable mask to indicate whether each ofthe channels is enabled, the per channel mask being generated based onvalues stored in each of a plurality of per-channel counters, each ofthe per-channel counters storing the per channel count value for eachrespective channel, wherein upon detecting that a particular channeldoes not satisfy a condition of a current unstructured control flowinstruction, the per-channel counter is to be updated with a tag addressidentifying one of the instruction pointer tags.
 2. The processor as inclaim 1 wherein the channel mask includes a single bit per channel,wherein a first binary value indicates that the channel is enabled and asecond binary value indicates that the channel is disabled.
 3. Theprocessor as in claim 2 wherein the channel mask bits are updated inresponse to the per-channel count values maintained for each channel. 4.The processor as in claim 3 wherein a per channel count value of 0indicates that the channel is enabled and wherein any other per channelcount value indicates that the channel is disabled.
 5. The processor asin claim 4 wherein the memory in which the instruction pointer tags arestored comprises a sorted linked-list memory structure.
 6. The processoras in claim 1 wherein each unstructured control flow instructioncomprises a predicate mask, the branch unit interpreting the predicatemask to determine whether each channel takes a conditional of theunstructured control flow instruction.
 7. The processor as in claim 6wherein if the predicate mask is uniform for all channels, then the perchannel count value is not updated but the execution instruction pointeris updated.
 8. The processor as in claim 6 wherein if the predicate maskis not uniform for all channels and if a current tag address is notfound in the memory, then the current instruction pointer tag isinserted into the memory.
 9. The processor as in claim 1 wherein theunstructured control flow instructions comprise diverging instructionsand converging instructions.
 10. The processor as in claim 9 furthercomprising: a sorted linked list memory structure used to storeinstruction pointers of converging instructions and/or diverginginstructions.
 11. The processor as in claim 10 wherein the unstructuredcontrol flow instructions include GOTO and JOIN instructions, the GOTOinstruction causing a jump to a first specified location in aninstruction steam and the JOIN instruction causing a return to a secondspecified location in the instruction stream.
 12. A method comprising:generating a tag address associated with a current unstructured controlflow instruction using at least a current instruction pointer;determining whether an instruction predicate mask of an unstructuredcontrol flow instruction is uniform for all channels of an executionunit, the predicate mask indicating which channels can process theunstructured control flow instruction type; searching a memory todetermine whether a stored instruction pointer tag corresponding to thetag address is stored therein, when the instruction predicate mask isnot uniform for all channels; and inserting an instruction pointer tagof the unstructured control flow instruction into the memory and savinga tag address-of the instruction pointer tag, when the instructionpointer tag is not found in the memory.
 13. The method as in claim 11,further comprising saving a tag address of the tag when the instructionpointer tag is found in the memory.
 14. The method as in claim 12further comprising updating a per channel count of a channel to includethe tag address.
 15. The method as in claim 12 wherein if theinstruction predicate mask is uniform for all channels, then updating anext instruction pointer.
 16. The method as in claim 15 wherein updatingthe next instruction pointer comprises setting the next instructionpointer equal to the sum of a current instruction pointer and a jumpoffset if all active channels can process the unstructured control flowinstruction type.
 17. The method as in claim 15 wherein updating thenext instruction pointer comprises incrementing the current instructionpointer if all active channels cannot process the unstructured controlflow instruction type.
 18. The method as in claim 14 further comprising:determining whether each channel is enabled or disabled and updating theper channel count only for those channels which are enabled.
 19. Themethod as in claim 18 further comprising: determining whether apredicate value indicates that a channel is enabled for the control flowinstruction type and updating the per channel count only for thosechannels which the predicate indicates are not enabled for that controlflow instruction type.
 20. A method comprising: comparing an instructionpointer address of a current instruction pointer of an unstructuredcontrol flow instruction to entries in a memory storing instructionpointer tags to determine whether the instruction pointer address isstored in the memory; saving a tag address of the current instructionpointer and comparing each per channel count maintained for each channelof an execution unit with the tag address upon determining that theinstruction pointer address is stored in the memory; and setting the perchannel count to a first value indicating an enabled channel if the perchannel count is equal to the tag address, wherein the first valuecomprises
 0. 21. The method as in claim 20 wherein if any channel isenabled for the unstructured control flow instruction then setting anext instruction pointer by incrementing a current instruction pointer.22. The method as in claim 20 wherein if no channel is enabled for theunstructured control flow instruction then setting a next instructionpointer equal to a current instruction pointer added to a jump offsetvalue.